This application claims the priority of German patent document 100 15 592.2, filed Mar. 29, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method of identifying the ignition stroke in a single-cylinder four-stroke engine, in which the position and the angular speed of the crankshaft are determined.
In a four-stroke engine, the induction stroke and the compression stroke, on the one hand, and the power or ignition stroke and the exhaust stroke, on the other hand, take place in successive revolutions of the crankshaft. In a carburetor engine, fuel is fed in the induction stroke together with the combustion air as soon as the intake valve opens, which is controlled by the camshaft. In contrast, in the case of an injection engine, the fuel is injected into the cylinder just before the start of the power stroke; that is, just before the piston reaches the top of dead center. In case, the recognition of the correct point in time for the injection of fuel represents a particular problem. Although, by measuring the position and the angular speed of the crankshaft, the correct point in time for the injection of the fuel prior to the ignition stroke can be determined, it cannot be determined in particular whether the crankshaft is carrying out that revolution in which the ignition stroke takes place.
Four-stroke engines are known in which fuel is injected into the cylinder during each revolution of the crankshaft, just before the piston reaches the top of dead center, and the spark plug is ignited once during each revolution of the crankshaft. This means that, during each second revolution of the crankshaft, fuel is injected at the wrong point in time and an ignition spark is generated, specifically during that revolution of the crankshaft in which the induction stroke takes place. Such an approach results in an increased burning-off of the spark plug and increased current consumption, which in turn can lead to problems with respect to the charge balance. However, it is a more serious disadvantage that very poor exhaust gas values occur because the establishing angle of the injection pulses cannot be emitted synchronously with the stroke.
Because the camshaft rotates at half the angular speed of the crankshaft, it could easily be used to determine during which of two successive revolutions of the crankshaft the ignition stroke is taking place. However, for this purpose, an additional sensor would have to be assigned to the camshaft to determine its position. This requires additional costs, and the accommodation of the generator wheel on the camshaft and of the pertaining sensor presents problems because of the limited available space.
It is an object of the invention to provide a method of the initially mentioned type which, without using a separate sensor to determine the position of the camshaft, permits the determination of that revolution (of two successive revolutions of the crankshaft) in which the ignition stroke takes place.
This and other objects and advantages are achieved by the method according to the invention, in which the time period from the top of dead center to a defined angle of rotation of the crankshaft is measured, and the measured periods of two successive revolutions are compared. The ignition stroke occurs during the revolution of the crankshaft with the shorter period duration.
The invention is based on the recognition that the angular speed of the crankshaft of a single-cylinder four-stroke engine is nonuniform. During the ignition stroke in particular, an accelerating force acts upon the crankshaft, while, during the exhaust, induction and compression stroke, a braking force acts upon the crankshaft by way of the piston and the connecting rod. Because of the resulting different angular speeds of the crankshaft, the period duration, that is, the time duration required for the rotation of the crankshaft about a defined angle of rotation during two successive revolutions of the crankshaft, will differ. By measuring and comparing the period durations of two successive revolutions of the crankshaft, it can therefore be determined definitively during which of the two revolutions of the crankshaft the ignition stroke is taking place.
The period duration is preferably measured from the top of dead center to the bottom of dead center of the crankshaft because, in this range, the largest differences occur with respect to the angular speed of the crankshaft. The difference between the measured period durations of two successive revolutions of the crankshaft is therefore particularly large and clear.
Since, during the measuring of the period durations, disturbance variables are superimposed, the measuring of the periods is expediently filtered. In this case, a different filtering time constant is used during the starting operation than during the normal operation of the engine.
In comparison to a conventional four-stroke injection engine, in which fuel is injected and an ignition spark is generated during each revolution of the crankshaft, the method according to the invention has the advantage that the useful life of the spark plug is doubled because only half the number of ignition sparks is generated. As a result of the reduced energy requirement of the ignition coil, the generator can be smaller and less expensive. In addition, the crude emissions are reduced because the fuel is injected correctly with respect to its phase, so that a less expensive (coating) catalyst can be used. This is achieved without the use of a sensor to determine the position of the camshaft, and the revolution of the crankshaft in which the ignition stroke is takes place is identified.